The present invention relates to battery powered implantable devices, and in particular to an improved Radio Frequency (RF) power amplifier for Implantable Cochlear Stimulator (ICS) systems. Such ICS systems provide improved hearing for the hearing impaired. The RF power amplifier serves the important function of providing power to the implanted part of the ICS system and the efficiency of the RF power amplifier is crucial in developing the miniaturized systems of the future.
U.S. Pat. No. 4,400,590 issued Aug. 23, 1983 for “Apparatus for Multi-Channel Cochlear Implant Hearing Aid System” describes and illustrates a system for electrically stimulating predetermined locations of the auditory nerve within the cochlea of the ear, which system includes a multi-channel intra-cochlear electrode array positioned adjacent to the auditory nerve within the cochlea of the ear. The electrode array comprises a plurality of exposed electrode pairs spaced along and embedded in a resilient curved base for implantation in accordance with the method of surgical implantation described in U.S. Pat. No. 3,751,605 issued Aug. 7, 1973 for “Method of Inducing Hearing.” The hearing aid system described in the '590 patent includes a signal processor located outside the body of a hearing impaired patient. The signal processor receives audio signals (i.e., sound waves) and converts the audio signals into analog data signals which are carried by a lead through the patient's skin to the implantable multi-channel intra-cochlear electrode array. The analog signals are applied to selected ones of the plurality of exposed electrode pairs included in the intra-cochlear electrode array to electrically stimulate predetermined locations of the auditory nerve.
U.S. Pat. No. 4,532,930, issued Aug. 6, 1985 for “Cochlear Implant System For an Auditory Prosthesis” describes and illustrates a multiple electrode system which does not employ a lead passing through the skin. Though multiple electrodes are employed to stimulate hearing, the system only operates with a single pulsatile output stimulating a single electrode channel at any given time. Such a sequential system is limited in speed of operation, and does not provide for analog operation where continuous stimulating signals, controllable in amplitude, are simultaneously applied to a number of electrode channels. Further, the system is subject to charge imbalance with misprogramming or circuit fault and inefficient use of electrical power. Moreover, once the stimulator unit is implanted there are no means for monitoring its ongoing circuit operation or power requirements so as to optimize its continued operation.
U.S. Pat. No. 4,592,359, issued Jun. 3, 1986 for “Multi-Channel Implantable Neural Stimulator” describes a cochlear implant system having 4 current sources and 4 current sinks per channel, controlled by series switches, to provide 16 different circuits for supplying 16 levels of 2 polarities to each output channel. In a pulsatile mode, the system provides for simultaneous update (amplitude control) and output to all channels. However, the system does not permit simultaneous analog update and output on all channels and the electrode pairs for each channel are not electrically isolated from all other electrode pairs whereby undesired current leakage may occur. Further, once the stimulator is implanted there are no means for monitoring its ongoing circuit operation or power requirements so as to optimize its continued operation.
U.S. Pat. No. 4,947,844, issued Aug. 14, 1990 for “Receiver/Stimulator For Hearing Prosthesis” describes and illustrates a multiple channel electrode system. The system includes an implantable receiver/stimulator connected to an implantable electrode array. Included in the implantable receiver/stimulator is a transmitter for telemetering one electrode voltage, measured during stimulation, to an external receiver for monitoring and analysis as an indicator of proper operation of the implantable stimulator. The transmitter comprises an oscillator operating at a frequency of about 1 MHz. The output of the oscillator is coupled to the implant's receiving coil. The oscillator signal, when received after transmission, is demodulated to recover the selected voltage waveforms.
Recently, ICS systems have been introduced which include a behind-the-ear speech processor, e.g., of the type described in U.S. Pat. No. 5,824,022 issued Oct. 20, 1998 for ‘Cochlear Stimulation System Employing Behind-The-Ear (BTE) Speech Processor with Remote Control.’ BTE speech processors offer several advantages, but because of their small size, BTE speech processors are limited in the size of the battery they may carry (which in turn limits the useful life of the battery.) The small battery size results in a requirement for very low power consumption. Further, known BTE external devices may be required to cooperate with two or more varieties of implantable devices. Such varieties of implantable devices have significantly different power requirements, and techniques such as dynamic output power control have been used to meet the varying power requirements. Unfortunately, known methods of dynamic output power control fail to produce consistent voltage output when required to operate over a wide range of voltages. Specifically, the threshold voltage of amplifiers with MOS transistors may limit the minimum voltage the amplifier can produce with adequate output voltage control.
What is needed in an RF amplifier with a wide dynamic range, suitable for use in an ICS system.